1. Field of the Invention
The present invention relates to a valve timing adjusting apparatus for adjusting timing (valve timing) of opening and closing one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
A conventional valve timing adjusting apparatus is known to include a housing serving as a driving-side rotor rotatable synchronously with a crankshaft and a vane rotor serving as a driven-side rotor rotatable synchronously with a camshaft. In the above valve timing adjusting apparatus, the housing includes shoes and the vane rotor includes vanes, and an advance chamber and a retard chamber are defined between the shoe and the vane arranged one after another in a rotational direction. By supplying working fluid to the advance chamber or the retard chamber, the vane rotor is rotated relative to the housing in an advance direction or a retard direction. As a result, a phase (engine phase) of the camshaft with respect to the crankshaft used for determining valve timing is adjusted (for example, JP-A-2002-357105 corresponding to U.S. Pat. No. 6,779,499).
In a valve timing adjusting apparatus described in JP-A-2002-357105, by engaging the vane rotor with the housing at the event of stopping and starting the internal combustion engine, the engine phase is held at an intermediate phase defined between a full advance phase and a full retard phase. Due to the above technique, even when the camshaft applies variable torque that attempts to rotate the vane rotor relative to the housing in the advance direction and the retard direction alternately, the engine phase is mechanically held at the intermediate phase at the event of stopping and starting the internal combustion engine, during which pressure of working fluid is relatively low.
The variable torque (torque reversal) is generated periodically such that the variable torque attempts to advance or retard the camshaft in accordance with the rotation of the internal combustion engine. For example, variable torque is caused by a spring reaction force of a valve spring of the valve that is opened and closed by the camshaft. Also, variable torque may be caused by drive reaction force from a mechanical pump in a case, where the mechanical pump is operated by the camshaft.
Also, in the valve timing adjusting apparatus described in JP-A-H10-252420 corresponding to U.S. Pat. No. 5,775,279, a torsion spring has one end connected with the driven-side rotor and has the other end connected with the driving-side rotor. More specifically, in the apparatus of JP-A-2002-357105, when the one end of the torsion spring is twisted relatively in the retard direction by the rotational force of the driven-side rotor, a restoring force in the advance direction is generated and applied to the driven-side rotor. As a result, the restoring force of the torsion spring quickly rotates the driven-side rotor relative to the driving-side rotor in the advance direction, and thereby responsibility is improved. Also, when supply of fluid is stopped, the restoring force of the torsion spring biases the driven-side rotor in the advance direction relative to the driving-side rotor. Therefore, even in a case, where variable torque from the camshaft is applied to the driven-side rotor, “fluctuation of the cam phase” that may cause noise is successfully suppressed, and thereby the engine phase is capable of being held at a full advance phase.
In JP-A-2002-357105, when the internal combustion engine is stopped, for example, the rotations of the vane rotor relative to housing in the advance and retard directions are limited by different limitation mechanisms, respectively, in order to reliably hold the engine phase at the intermediate phase, and in order to adjust the engine phase or valve timing suitable for the operational state of the internal combustion engine. Each of the limitation mechanisms includes a control pin that is actuated by fluid, and the control pin is assembled into the vane rotor. Because the control pin of each of the limitation mechanisms is actuated independently, a flow channel structure is complicated accordingly.
In contrast, the restoring force of the torsion spring is caused by torsional torque in the apparatus of JP-A-H10-252420. When rotational force in the advance direction twists the connection end portion of the torsion spring, which is connected with the driven-side rotor, the restoring force outputted to the driven-side rotor against relative rotational force is increased. Restoring force of the above torsion spring provides torque that attempts to hold the engine phase at an intermediate phase. However, it is difficult to keep the engine phase at the intermediate phase by using the restoring force of the torsion spring.
Also, even in a case, where the restoring force of the torsion spring is used to bias the engine phase to the intermediate phase in the event of starting the internal combustion engine or in the later event of stopping the supply of working fluid, the restoring force is required to be set at a magnitude equal to or greater than certain torque such that the restoring force advances or retards the engine phase against the average torque of variable torque. The restoring force characteristic of the torsion spring for ensuring the above certain torque causes an increase of torsional torque per unit change of the phase. As a result, when a tracking control for causing the engine phase to follow the target phase or another control for limiting the engine phase within the intermediate phase region is executed, the change amount of the torsional torque required to reduces the gap between the engine phase and the target phase (intermediate phase) is substantially large. Therefore, controlability may deteriorate, and thereby it may become difficult to accurately adjust the engine phase to the target phase.